Semiconductor devices are typically formed in arrays as a plurality of regions in semiconductor wafers. After a sequence of processing steps which complete the devices as integral parts of the wafers, the wafers are severed to yield a plurality of individual devices. The devices are then typically tested and assembled into protective device housings in subsequent operations such as device mounting, lead bonding and packaging operations.
These latter operations typically are sequential operations in contrast to the wafer processing operations which are generally batch-type operations. Sequential operations tend to be more costly than the batch-type operations. However, some economies are obtained by handling techniques which allow the individual devices to remain in supported arrays until the devices become ultimately mounted in individual device packages. Test data on such arrayed devices, even though obtained by sequential tests on the devices, are recorded as arrayed data to correspond directly to the array positions of the tested devices in the arrays. The arrayed test data are used to direct sorting operations, as, for example, to cause the removal of devices having only certain acceptable parameters and to leave untouched all other devices of the original array.
A technique for preserving the array of devices in a wafer after such wafer is severed into individual devices is to mount the wafer onto an adhesively coated film-type diaphragm which is stretched across an opening of a temporary mounting frame. The wafer is then typically sawed along orthogonal boundary lines between the defined devices. The depth of the saw cut is adjusted to slice through the thickness of the wafer without damaging the underlying film-type material of the diaphragm. The individual devices remain attached to the diaphragm after the sawing operation is completed.
As an example, a particular, temporary mounting frame is made of a flat, rectangular metal sheet having a central, circular opening. The diaphragm is a commercially available adhesively coated polymer film which is stretched across the opening and is adhesively attached to the coplanar metal surface of the frame surrounding the opening. A wafer may then be mounted to the adhesively coated film surface in the opening of the frame.
Individual devices which are supported by such a film-type diaphragm may be removed in a typical sorting operation by commercially available sorting apparatus. In a typical sorting operation, the mounting frame is indexed in the plane of the array of devices to align a device selected for removal with a transfer station.
At the transfer station a pushpin is urged from beneath against the selected device, generating stresses between the adhesive coating on the top surface of the diaphragm and the underside of the device, and urging the device out of adhesive contact with and away from the diaphragm. A vacuum probe which is movably mounted above the array moves from above the selected device into contact with its top surface, generating a vacuum hold on the device and carrying the selected device away from the diaphragm.
According to one technique, the pushpin, which is urged against the underside of the device, is pointed. During its upward movement the pin actually pierces the diaphragm and pushes directly against the underside of the selected device. The resulting motion is a positive pulling motion by the device on the adhesive coating to lift the device off the diaphragm. The diaphragm piercing technique is a frequently used technique which works well with comparatively small devices, such as devices which are in a range of a quarter of a millimeter along each edge.
A problem with using the diaphragm-piercing pushpin is related to a vacuum which typically holds down the diaphragm against the upward directed force initiated by the pin. Piercing the diaphragm with the pin appears to have an unfortunate side effect of extending the vacuum from below the diaphragm to the space between the upper surface of the diaphragm and the base of the device. Such a vacuum tends to slow the separation of the device from the adhesive coating. Also, the piercing pin does not appear to sufficiently flex the diaphragm of larger devices to initiate separation of the selected device from the diaphragm at reasonably desirable speeds. Particularly larger devices appear to resist separation from the diaphragm. A stronger upward push by the pin typically alleviates the problem but tends to raise the force exerted against the devices to an undesirably high value at which some devices become damaged.
Another technique of removing selected devices from the adhesively coated diaphragm involves a pin having a blunt tip, as, for example, a rounded tip. Such a tip does not pierce the diaphragm. Instead it merely pushes against the underside of the diaphragm, thereby flexing the surface of the diaphragm to such an extent with respect to the inflexible adjacent surface of the device that the adhesive coating peels from the underside of the device to release its hold on the device. During the upward movement of the pin during which the peeling takes place, the selected device is raised above the plane of the other devices in the array and moves into contact with the vacuum probe.
A problem with such a non-piercing transfer technique is that at times the adhesive coating tends to peel faster on one side of the device than on the other. As a result, devices tend to tilt and remain at least partially in contact with the diaphragm. The tilted devices cannot be contacted squarely by the vacuum probe and are consequently not removed from the array. The problem of having devices in an array tilt can be alleviated by slowing down the upward movement of the pin. Apparently a slow upward movement of the pin allows more time for the adhesive to peel more uniformly from the selected device. Unfortunately, such slowed down movement of the transfer apparatus also results in a less efficient and hence more costly transfer operation.